refactor: extract SceneThumbnailStrip out of MainWindow.xaml — Phase 2, Commit 16

SceneThumbnailStrip.xaml(.cs) takes the Row 0 scene-picker strip (ItemsControl
over Scenes with staged/live border highlight); Thumbnail_MouseLeftButtonDown
moved into the control, transitioning via DataContext.TransitionToScene.

Zero behavior change; build 0 warnings; 246 pass, only the 2 known failures.
Docs: ViewModels/index.md Phase 2 tracker updated in the same commit.

Controls/SceneThumbnailStrip.xaml.cs

@@ -0,0 +1,21 @@
+using System.Windows;
+using System.Windows.Controls;
+using System.Windows.Input;
+using ytLive.Models;
+using ytLive.ViewModels;
+
+namespace ytLive.Controls;
+
+public partial class SceneThumbnailStrip : UserControl
+{
+    public SceneThumbnailStrip()
+    {
+        InitializeComponent();
+    }
+
+    private void Thumbnail_MouseLeftButtonDown(object sender, MouseButtonEventArgs e)
+    {
+        if (sender is Border { DataContext: Scene scene } && DataContext is MainViewModel vm)
+            vm.TransitionToScene(scene);
+    }
+}